Gamma-Ray Assay of Low-Enriched Uranium Waste

A method based on the detection of 0.767- and 1.001-MeV gamma rays from ^234mPa has been developed for the assay of ²³⁸U in large containers of uranium waste. Detailed calibration and assay procedures were obtained for × 4- × 4-ft plywood boxes of combustible waste. The gamma rays were detected with a large NaI crystal and a line-source “standard” box was used for calibration. The calibration was extended over a wide range of box weights using Monte Carlo calculations of gamma-ray attenuation. The error in the calibration is <6% (2σ); much larger assay errors can result from heterogeneities in the waste and from the age dependence of the ²³⁸U daughters. The detection limit for a 5-min count is about 30 g ²³⁸U in a typical box of combustibles. Data generated for the box problem, together with additional Monte Carlo calculations, were used to devise a simple analytical model applicable for the assay of boxes and cylinders in a range of practical geometries. The essential feature of this model is a flux buildup factor which accounts for Compton-scattered photons.